Volume phase holograms, i.e., refractive index modulated holograms, are well known and have been prepared from a variety of photosensitive materials, particularly ones in which the holographic fringes are formed by photopolymerization or cross-linking.
Dichromated gelatin (DCG) has been widely used for the formation of volume phase holograms. The mechanism by which the holographic image is formed in DCG, however, is not settled, and several mechanisms have been proposed. One such proposal is that "cracks" or "voids" are formed between fringe planes in the gelatin, and that the resulting difference between the index of refraction of air (in the cracks or voids) and that of gelatin gives increased index modulation. (See A. Graube, "Holographic Optical Element Materials Research", U.S. Air Force Office of Scientific Research Technical Report 78-1626 (1978), available from the U.S. Defense Technical Information Center as DTIC Technical Report ADAO62692, particularly pages 95-113.) The presence of "voids" is disputed in an earlier article (R.K. Curran and T.A. Shankoff, "The Mechanism of Hologram Formation in Dichromated Gelatin", Applied Optics, July, 1970, Vol. 9, pp. 1651-1657; particularly p. 1655), which concludes that "cracks" provide an air-gelatin interface. A 1980 publication (B.J. Chang, "Dichromated Gelatin Holograms and Their Applications", Optical Engineering, Vol. 19, pp. 642-648 see particularly pp. 642-643) proposes yet another mechanism: a "molecular-chain spring" model. Yet another mechanism--an interaction between isopropanol and gelatin causing cracks when the isopropanol is removed--is proposed by J.R. Magarinos and D.J. Coleman, "Holographic Mirrors", Optical Engineering, 1985, Vol. 24, pp. 769-780. These researchers recognized that one way to support the hypothesis of the presence of voids (microvoids) would be to demonstrate the loss of holographic efficiency by replacing air in such voids with a liquid having an index of refraction matching that of gelatin, but they differed as to whether they achieved such a result. It is clear, however, that these researchers were investigating mechanisms rather than ways to modify the optical properties.
A particularly useful photopolymerizable composition to form volume phase holograms is described and claimed in U.S. Pat. No. 4,588,664 issued May 13, 1986 to Herbert L. Fielding and Richard T. Ingwall. These photopolymerizable compositions comprise a dye sensitizer such as methylene blue, a branched polyethylenimine as a polymerization initiator, and a free radical polymerizable ethylenic unsaturated monomer, e.g., an acrylic monomer, and preferably, lithium acrylate. A method of stabilizing holograms formed from this photopolymerizable composition is described and claimed in U.S. Pat. No. 4,535,041 issued to Herbert L. Fielding and Richard T. Ingwall on Aug. 13, 1985.